This invention relates to polyimide angularity enhancement layers in liquid crystal displays which enhance the viewing angle of liquid crystal displays. Novel polyimide copolymers are disclosed. Also, multilayer angularity enhancement constructions comprising polyimides are disclosed.
Negative birefringent films greatly enhance the image quality of liquid crystal displays viewed at off-normal angles by compensating for image degradation due to xe2x80x9cnaturalxe2x80x9d net positive birefringence of the liquid crystal materials. Typically, negative birefringent films have been prepared by precision stretching of polymer films or by precisely controlled vapor deposition of thin ceramic layers. However, practicioners have found that precise control of film stretching in order to exactly control the resulting birefringence is difficult, and that ceramic films can be unstable and are not practical for large surface areas.
Non-stretched negative birefringent polyimide films are known. U.S. Pat. No. 5,344,916 describes polyimide films prepared from benzene dianhydrides and/or bis(benzene)dianhydrides and monoaromatic and/or polyaromatic diamines. While the polyimides are generally soluble, some polyimides are soluble only in solvents that are incompatible with certain useful liquid crystal polymeric substrates onto which the polyimides could be coated. The monoaromatic and/or polyaromatic diamines are linked benzeneamine compounds (benzidine derivatives, bis(4-aminophenyl)methane derivatives, 2,2-bis(4-aminophenyl)propane derivatives, and the like) rather than polyaromatic diamines such as 9,9-bis(4-aminophenyl)fluorenes. Many linked benzeneamines must be handled with care due to their suspected human toxicity.
Homopolymers of 9,9-bis(4-aminophenyl)fluorenes and aromatic dicarboxylic acid dianhydrides are known. U.S. Pat. No. 4,897,092 describes the preparation and properties of polyimide membranes prepared from the homopolymers, the membranes being useful for gas separations. The membranes are described as being approximately ten times the thickness of layers useful in liquid crystal displays, and optical properties of the membranes are not elucidated. No copolymers of 9,9-bis(4-aminophenyl)fluorenes and aromatic dicarboxylic acid dianhydrides with a non-fluorenyl aromatic diamines are described. French patent application number 2,650,829 discloses a number of possible uses for homopolymers of 9,9-bis(4-aminophenyl)fluorenes and aromatic dicarboxylic acid dianhydrides, including alignment films for liquid crystal displays. No copolymers with additional non-fluorenyl aromatic diamines are described, and no indication of optical compensation properties of the films is presented. Copolymers of 9,9-bis(4-aminophenyl)fluorenes with two or more aromatic dicarboxylic acid dianhydrides are described in U.S. Pat. No. 4,845,185 and in Japanese patent application number 62292836 (Derwent abstract). Polyimides described in U.S. Pat. No. 4,845,185 are shown to be solvent soluble and to have high heat resistance, but are only moderately light transmissive. Copolymers of 9,9-bis(4-aminophenyl)fluorene, an aromatic tetracarboxylic acid dianhydride and an aliphatic diamine are disclosed in Japanese patent application number JP 63295633. Polyimides described therein are said to have good thermal resistance and workability, and find use as electric or electronic materials (Derwent abstract), but no optical properties are described.
Japanese patent application 5-31341 discloses a polyimide gas separation membrane which is a copolymer of 9,9-bis(4-aminophenyl)fluorene and aromatic dicarboxylic acid dianhydride with a non-fluorenyl polycyclic fused ring aromatic diamine. No optical properties of these copolymers are disclosed.
Briefly, this invention provides a negatively birefringent angularity enhancement layer in a liquid crystal display, which display comprises a liquid crystal cell, wherein the negatively birefringent angularity enhancement layer comprises a polyimide comprising a plurality of pendant fluorene structural units and preferably is disposed on at least one surface of the liquid crystal cell.
In another aspect, the invention provides a liquid crystal display comprising in sequence a first polarizer, a liquid crystal cell, and a second polarizer, and, disposed between said liquid crystal cell and at least one of said polarizers, an angularity enhancement construction comprising a colorless, optically-transparent negatively birefringent polyimide layer comprising the reaction product of a 9,9-bis(aminoaryl)fluorene and at least one aromatic tetracarboxylic acid dianhydride, and optionally one or more aromatic diamines. Preferably, the aromatic diamine has one six membered ring, more preferably it is a benzene diamine. Preferably, the polyimide layers exhibit an out-of-plane birefringence value of from about xe2x88x920.001 to about xe2x88x920.2, preferably xe2x88x920.001 to about xe2x88x920.04.
In yet another aspect, the present invention provides a multilayer angularity enhancement construction comprising an optically transparent, optically isotropic substrate having on at least one surface thereof at least one layer of a polyimide, preferably the total thickness of the polyimide layer or layers being greater than or equal to about 1 xcexcm and/or the polyimide layer exhibiting an optical retardation of at least 25 nm; on the surface of the at least one polyimide layer, there is disposed one or more layers of optical adhesive; and, as the outermost layer(s) of the construction, on the adhesive is a layer that acts as both release liner for the optical adhesive and as a protective layer for the construction. See FIG. 1. Preferably, the optically transparent, optically isotropic substrate is a polymer; most preferably, the substrate is poly(methyl methacrylate), PMMA.
In a further aspect there are provided novel copolymeric polyimides comprising the copolymerization product of a 9,9-bis(aminoaryl)fluorene, at least one aromatic tetracarboxylic acid dianhydride, and an aromatic diamine free of fused rings.
In this application:
xe2x80x9calkxe2x80x9d as in xe2x80x9calkylxe2x80x9d and xe2x80x9calkoxyxe2x80x9d means an aliphatic group or moiety having from 1 to about 20 carbon atoms;
xe2x80x9carylxe2x80x9d means an aromatic group or moiety having from 6 to about 20 carbon atoms;
xe2x80x9cgroupxe2x80x9d means the specified moiety or any group containing the specified moiety (as by substitution or extension) that does not adversely affect the composition;
xe2x80x9clayerxe2x80x9d means a coating on a substrate or a self-supporting film;
xe2x80x9cfilmxe2x80x9d means a generally thin, flexible self-supporting material;
xe2x80x9cretardationxe2x80x9d means the multiplication product of the refractive anisotropy and the film thickness, which is a physical quantity that equals the phase difference of light as it passes through the film; this value changes with the viewing angle; and
xe2x80x9cwater-whitexe2x80x9d means colorless and having a light transmission in excess of 90% in the wavelength range from 400 to 700 nm.
Polyimide films of the invention, based upon 9,9-bis(aminoaryl)fluorenes, exhibit advantageous optical properties and certain processing advantages based upon the three-dimensional structure of the polymer from which they are formed. Both physical and optical properties of the films can be tailored by appropriate selection of monomers, and it is desirable to strike a balance between them. To be useful in a liquid crystal display, films should be essentially colorless and, to be easily coated in thin layers, the polymer should be soluble in common, high-volatility solvents, i.e., appears to the naked eye to be a homogeneous solution. In polyimides, these two factors are usually detrimental to increased negative birefrigence, so a balance must be found.
This invention provides a novel angularity enhancement layer in a liquid crystal display comprising a liquid crystal cell, wherein the angularity enhancement layer is negatively birefringent and comprises a polyimide having a plurality of pendant fluorene structural units. When coated or cast as films, polyimides useful in the invention present an in-plane molecular orientation, which gives the film anisotropic optical properties.
Polyimides of the invention have high negative birefringence, in the range of xe2x88x920.001 to xe2x88x920.2, preferably xe2x88x920.001 to 0.04, more preferably xe2x88x920.001 to xe2x88x920.032, enabling them to be used as thin coatings, preferably 10 micrometers or less, in a liquid crystal display. Such a thin coating can be a layer on a flexible medium. Liquid crystal displays desirably are flexible and are very thin in order to reduce their overall weight. Thin layers of polyimides of the invention can be optically transparent and they can be water-white, slightly yellow, or yellow. Preferably, they are water-white.